Mining equipment has long been a key to economically successful mining operations. Originally, underground mining of thicker seams was the primary focus for coal-mining enterprises. With significant depletion of thick seams and additional focus on the safety and health implications of such mining, more attention has been concentrated in recent years on mining thinner seams of coal in both surface and underground mining operations. To accomplish better economics, surface miners uncovered as much overburden as was economically feasible for a given thickness of coal and then augured underground into the exposed high wall, recovering additional tons without the expense of removing the overburden. Until recently, these were round augers limited to short underground penetration distances. The advent of the high wall miner using a continuous underground mining machine in front of a conveyor system has allowed significant improvement in penetration depths. The major drawbacks in all auger and high wall mining operations have been localized roof falls and leaving enough space between holes to prevent collapse of the complete hill structure. Leaving enough coal to prevent collapse is extremely inefficient in that substantial pockets of coal remain after the mining operation, and there are possibilities of later subsidence into the bore holes in an uncontrolled and unscheduled manner. The aforesaid inefficiency of conventional high wall mining is particularly significant in instances where the surface of the seam follows a curvilinear path, such that only an extremely small fraction of the seam can be retrieved when employing spaced surface holes.
Efforts have been made over the years to improve recovery rates achievable by high wall mining apparatus. In that respect, different cutter head designs have been developed to achieve improved cutting action. There have also been developments in improving power systems and equipment reliability to reduce down time of high wall mining equipment.
Also, efforts have been made to expand the application of high wall miners beyond traditional applications by effecting higher and wider cuts. In this respect, cutter heads with larger diameters have been developed, together with larger motors, increased conveyor speeds, and the requisite interface equipment. Another more sophisticated approach has been the development of equipment that initially cuts a conventional bore and then proceeds to ream the bore to slightly larger dimensions. In this latter respect, a bore is made horizontally, normally with conventional cutting apparatus. In most instances, these high wall miners cut substantially greater quantities of minerals during the entry phase, with the hole size being only slightly augmented by reaming cutters. In this respect, various types of cutters have been developed for the reaming action, which are normally contracted during the conventional cutting and subsequently expanded during the return cutting operation. In other instances, the main cutter may be pivoted or otherwise minimally offset from the hole produced during the entry phase to produce the retreat reaming cut.
These combined cutting and reaming machines achieve only minor productivity advancements in relation to the complexities and disadvantages that are involved. In most instances, the cutters for the reaming operation are normally located rearwardly on the machine from the entry phase cutters. This, of course, presents the possibilities of a roof collapse, which can trap the mining equipment underground. As a result, limitations are frequently applied to the extent of the cut made by the reaming cutters, such as to minimize the possibility of roof collapse. These systems also have the disadvantage that even though round holes are cut, frequently there remain potential subsidence problems years after mining operations are completed. Thus, developments in high wall mining equipment over recent years have, for the most part, involved refinements to existing equipment and methods.